There is a dungeon master who runs the games with the goal of challenging players with wacky situations where there is always a chance of failure and character death.

The player characters are powerful - yet absolutely overmatched by a much larger and much more uncaring world. In game terms, PCs are limited to approximately levels 3 to 5 of 5th edition. In story terms, think Conan against Thulsa Doom.

Balance can go out of the window with a kitchen sink looking for a kitten. In game terms, the PCs can run into a giant 32HD stork or a 1hp mad wizard hermit crab cube, and it doesn't matter. Every encounter can play out with words, trickery, combat, running or a goat.

The Game Is Incomplete. Edge cases will arise and be dealt with lovingly.

Add 3 to 4 special starting abilities depending on your chosen class and type (e.g. soul magicians have different abilities). Or make them up.

If a player wants to use a standard 5E character, have them generate it whichever way suits you at 3rd level. After that, they gain experience as other characters in WTF.

1. b. Experience and "Levelling Up"

At the end of every game session, the player rolls on the class experience table (e.g. this) and their character acquires a new trait or ability. Each result should be used only once, so re-roll duplicates.

Optionally, make a custom list.

Optionally, complex quests or dungeons should have bonus boons and experience results as part of their "treasure".

1. c. Other Stuff is Optional

Personality and backgrounds. If a player wants to add this stuff to their character, it's fine.

Feats - if players really want them, let them add them on a roll of 20 for experience at the end of the session. Or as per the class tables.

Inspiration is optional.

Alignment is _really_ optional.

1. d. Important Notes

Any class can cast spells if they have learned them (possible as adventure rewards or from scrolls), but unless they have spell slots (acquired as class level-ups), they have to use hit points or ability points to cast them.

Any character can use any armour, but if they do not have a proficiency in it, they have a disadvantage on physical attacks, skill checks and must make saving throws to cast spells.

Any character can use any weapon, but if they do not have proficiency in it, they do not add their proficiency bonus to it.

2. Adventuring and Combat

Players get together to play a party of characters out doing stuff together. This means:

Player characters are presumed to get along initially.

Minimum time should be wasted getting new characters into the action. Suspend disbelief or just have them pop into existence, if required.

Replace dead characters with other playable options fast.

Players can do stuff. Anything. The general process for gaming is as follows:

DM describes scene

Player says what character wants to do.

If this seems reasonable, it happens. If there would be a complication, the DM offers it as a "yes, but" option. If nobody knows what the outcome would be, the dice will speak.

It is always presumed that combat is only one optional form of conflict resolution.

2. a. Ability Scores and Stuff

Scores and modifiers are as in 5th edition D&D (see p. 173 of the PHB).

Advantage and Disadvantage are cool mechanics and are used.

Ability checks, the standard DCs presented on p. 174 can be used. In opposed checks, a tie means no change to the situation (i.e. defender wins).

Optionally, to test ability, a 2d10 or 2d12 roll below ability may be called for. Proficiency bonus and skill bonuses can modify the ability for this test.

Skills can be from the general 5E list, or made-up for the requirements of the character.

Saving throws are used like in 5E (see p. 180).

The general rule remains: avoid rolling dice and prefer description and action over rolls.

2. b. Encumbrance & Inventories

Encumbrance and inventories in most games are a hot mess. The goal is to keep things simple.

Track item masses in "stones" or "inventory slots" or whatever. Each character can carry a number of stones equal to their Strength score. Each item above that number applies a cumulative -1 to all physical ability checks.

Each item large enough to be considered encumbering is assumed to mass one "stone". So, a suit of armour is one stone. A weapon is one stone. A golden statuette is one stone. A pack of rations is one stone. A rope is one stone. A quiver of arrows is one stone. A bundle of torches is one stone.

A regular, unarmored hireling is assumed to be able to carry 9 stones (1 stone is reserved for their personal belongings and clothes).

Small stuff can be lumped together as one single "personal effects" item weighing 1 stone (e.g. a pouch with coins, gems, shaving kit, flint and steel, old magazine, string, lead miniature, three dice and a pencil).

Small characters can carry less, large characters can carry more.

2. c. Time

The goal is to keep things simple. Generally the aim is to keep things simple. Different scenes happen at different time scales:

Overland travel happens in days and each day a group can carry out around three actions. The actions can be travel, explore / navigate / map / track, forage or rest. Going without a rest action is bed. A spell or effect that lasts a day, lasts through all these actions.

Note: In WTF games I assume the gritty realism variant (DMG p. 267), where a short rest is 8 hours and a long rest is 7 days.

Exploration of a ruin, say, happens in hours and each hour a group can carry out around three actions which can include searching rooms, mapping / tracking, or having encounters. Of course, use common sense.

Encounters happen in short minutes and each encounter might include talk, movement (flight) or combat.

A combat happens in seconds. The time rules on PHB p. 189 are fine, for all I care. But with the long rest times, avoiding combat is generally a good idea.

2. č. Resources

The core problem of time is that it consumes resources. Tracking time and reducing resources is important to keep players challenged and give them meaningful choice, but ... detailed tracking time and resources is a complete pain, so in WTF players use resource dice. There are two options:

Simple: Roll a die (usually a d12) after every scene where a resource was used, on a 1 it has run out.

Dice Chain:

d8 - Very abundant resource. Roll after scene > on 1, drop down to d6.

d6 - Normal resource. Roll after scene > on 1, drop down to d4.

d4 - Scarce resource. Roll after scene > on 1, it runs out.

With a dice chain, avoid giving higher dice than d8, as this means that a resource will effectively never run out. A d8 should be reserved for something like an ancient wand, that can never be recharged. With a d6, a resource should be usable at least twice and have a decent (~50%) chance of giving 7 uses before running out. The great benefit of a dice chain for resources is that a player can tell when a certain resource is on the verge of giving out.

Ammunition - bullets or other ammunition that requires reloading - d6 - roll after every combat turn. This means you can end up having to reload (spend an action) during a combat. The plus is that a full ammo box (1 stone) might include enough ammo for several reloads.

Charges - purchased wands - d6 - after every use. Wands are also awesome, because they're small and don't take up inventory space!

Charges - irreplaceable ancient crap - d8 - after every use.

It goes without saying that resources are fragile and casual fireballs or lightning bolts may destroy them.

2. d. Movement

Speed is treated as an ability score, with attendant bonuses. For chases, you can just do opposed ability checks between quarry and pursuers. If the quarry wins, they pull away, if the pursuers win, they close. Use the slowest member's ability. Of course, a slow member can be dropped behind, giving the rest of the party a re-roll. Leaving behind a slow, over-encumbered dwarf to be eaten by the blugger-beast may be a viable solution. Or a knee-capped ex-henchperson.

Per travel action (remember, up to three per day), a character can move its speed score in miles in good conditions. Or, you know, enter about 2 road hexes with 6 mile hexes.

Stealthy travel is at half speed. That's fortunate, because careful tracking is also at half speed. So the chase stays on! But slowly! And with food running out!

Distance

In combat WTF doesn't use miniatures and squares. Space is abstracted to four distances.

Contact (~ 0 feet) - aka. Melee Range: the combatants are locked in close combat. A disengage action is required to move away into close range without provoking an opportunity attack. An encounter only starts at this distance if all parties are surprised.

Close (~ 30 feet) - aka. Throwing Range: the combatants can throw ranged weapons at each other or charge into melee. If an enemy disengages to close range, it is possible to charge after it and attack again on your turn. An encounter can only start at this distance if one of the parties is surprised. Attacking at this distance, ranged weapons automatically win initiative against melee.

Medium (~ 60 feet) - aka. Bow Range: the combatants are too far apart for anything except bows or pistols ... or speech. Thrown weapons are at disadvantage. Unless both sides commit to melee combat, the distance is too long to charge into battle. If one side chooses to retreat, they can keep their distance. Charging against an entrenched enemy at this distance gives them a free ranged attack and advantage on initiative in the first melee round.

Long (~ 120 feet +) - aka. Shouting Range: the combatants are too far for meaningful interaction. All ranged attacks are at disadvantage. Shields give resistance to even those ranged attacks that do hit. The range is too great for thrown weapons to be used effectively. The distance is too long to even charge one another. If one side chooses to retreat, they can keep their distance or escape with advantage on their speed check. Charging against an entrenched enemy at this distance gives them two free ranged attacks and advantage on initiative in the first melee round.

The general modifiers from the PHB can apply:

Difficult terrain doubles the time required to cover a given distance.

Dropping prone is a free action.

Crawling doubles the time required to cover a given distance (and crawling over difficult terrain quadruples it).

Getting up takes an action (standard or movement) - this is different from the PHB, which says that getting up takes movement equal to half your speed. But, due to the simplified movement in WTF, this doesn't really come into play. This also means, that if you're knocked prone, running away will be hard.

Running Away from Melee

Turn 1 - disengage or flee from melee to close range

Turn 2 - at close range flee = opposed speed checks against pursuers.

This can be hard, if you lose initiative against an enemy! A defensive retreat can help here (giving resistance to damage).

2. e. Encounters & Combat

Combat is not an encounter, it is a resolution to a conflict in an encounter. Other options are talking, or stealth, or trickery, or running away. Combat is dangerous.

Surprise happens if during random encounters or while wandering around carelessly and walking into another group of creatures or when walking into an ambush: A player rolls 1d6 for the party, on a 1-2 they are surprised. Same goes for opponents (unless it's an ambush). Elf-type characters are only surprised on a 1.

Unsurprised characters get a free round.

Initiative - each side rolls 1d20 each round, higher number goes first. On ties, chaos reigns, and all actions happen at once, while damage and disabilities only resolve at the end of the round (so it's possible for a wizard to fireball 10 orcs, who also kill her with arrows). Initiative is rerolled each round.

Dodge / Defensive fighting - focus on defence, gives you advantage on dexterity saves and attacks from opponents you see have disadvantage.

Help - help another character, this gives advantage on their next check. E.g., you can help the Fighter trip the Ogre by sliding the dead Slothrog behind the ogre.

Hide - well, if you're not seen, you can hide.

Ready - "I ready my lance to poke him in the eye when he gets closer".

Use an Object - "The Thief Activates the Eye of Submission Against the Lover of the Ogre Queen."

This is some more standard stuff:

Big enemies have reach - this means they can hit enemies closing for melee as a reaction.

Ranged attacks and spells in melee are at a disadvantage.

Firing into melee - on a miss, roll to hit the closest ally in the same melee.

Two-weapon fighting - with a second weapon, or shield, you can make a bonus attack (lose shield defense bonus) without ability modifier to damage.

Grappling - attack with Athletics or Acrobatics skills, then you can move and drag at half speed. It can use its action to succeed in an Athletics or Acrobatics check against you.

Shoving a creature is the same, except you don't hold onto it and it moves 5 feet or falls prone.

Cover gives AC bonuses up to +5.

Resistance halves damage, vulnerability doubles damage.

This stuff is different.

Critical hits double all damage (dice and modifiers) and critical hits explode (so a second 20 turns it into a x4, a third into x8, etc.).

A character hit by a critical hit has the effectivenes of their armour reduced by 1 until it is repaired.

Shields shall be splintered (so shall swords) - a shield can be used to negate incoming damage (but this destroys the shield) or a weapon can be sacrificed to reduce incoming damage in half (this also destroys the weapon).

On 0 hp a character must make a Con save against a DC = damage taken below 0. On a failed save, the character is dead. On a successful save, the character is knocked out.

When combat ends, the knocked out character must make a save again. On a failed save, the character is dying and requires serious medical assistance. On a successful save, the character regains consciousness and is at 1hp.

In any case, the character gets to roll on a gruesome damage table.

2. f. Resting

Short rest = 8 hours. Spend Hit Dice + Con. mod to regain hp.

Long rest = 7 days. Regain all hp. Regain 2 spent Hit Dice.

3. Magic

Magic in WTF is inherently horrible and scary and involves the breaking of the rules of reality, the opening of cataclysmic channels to the cosmos and puffing the magic dragon. It's not for the faint of heart.

Premises:

Magic in D&D-type games is fun and all characters should be able to do it!

Magic should lead to weird unforeseen side-effects, because it should.

As it's default WTF uses the standard Wizard-style spell-casting from 5th edition D&D, so spells known and spell slots. The differences are as follows:

All classes can cast spells, but if they don't have the spell slots for them (because the spells are too high level or whatever), they pay for them in hit points or ability score points.

Wizards can pay for these extra spells at a cost of 1hp or 1ability point per level.

Thieves pay 2hp or 2 ability points per level.

Fighters pay 3hp or 3 ability points per level.

All classes can potentially learn all the different spells.

There are no specific cleric classes. Clerics are, after all, just fancy wizards who believe in demons.

Concentration works as standard (see PHB p.203), except that a wizard can "lock" or "reserve" an extra spell slot for a given spell, to keep it running in the background, so concentration is not lost from damage or incapacitation or casting a second spell.

Spell slots can only be acquired for spells up to level 4, spells above that level are always paid for in blood.

Cantrips beyond available spell slots are paid for in blood as though they were first level spells, because they're badly assembled and when you do blood magic, the outer chaos always takes more than it gives.

3. a. Alternate Magic Systems

Alternate and weird magic systems are absolutely possible and encouraged, from the pictomancer who creates spells by painting them (thus requiring pigments, tools and a substrate to cast at all) to the astrolomancer, who casts spells with a deck of cards (and can't cast magic without cards) to the blue wizard (who can only cast blue-themed magic) to the various magic implements-as-characters who require a secondary mind-dominated-henchman to wield them.